The Bible specifically calls attention to the roles of men and women in marriage, stating specifically that a man is to be the head of the house. Therein lies the issue because you cannot properly fit into this mold with two men or two women operating together within a marriage. The Bible is also clear when it establishes in Genesis that a woman is to be one flesh with a man within the parameters of marriage, and gives the command to be fruitful and multiply. LGBT couples simply cannot perform in roles that were so clearly established for a man and a woman.